is 350 MHz. The clock rate of 2.5GBase-T
is 200 MHz, translating to 625 Mbits/sec
on each pair after the 64B/65B encoding
and PAM 16 modulation. Similarly, the
clock rate of 5GBase-T is 400 MHz, resulting in 1250 Mbits/sec per lane with
the same encoding and modulation.
Because of the limitation in bandwidth
for Category 5e for 5GBase-T operation,
the PSD (power spectral density) mask
is shaped differently for 2.5GBase-T and
5GBase-T. It allows for more power at
lower frequencies for 5GBase-T to overcompensate for the lack of bandwidth.

The 2.5GBase-T mask is lower to reduce the possible effects of crosstalk on
a 5GBase-T signal on the same installation. The encoding increases transition
density by mapping the possible 264 possibilities of bit streams to the more sig-nal-integrity-favorable sequences of the
265 bit streams. While you lose that 1/65

headroom, the signal-integrity advan-tages are worth it. The pulse amplitudemodulation (PAM) of 16 levels increasesthe baud rate by 4 at the same trans-mission speed. The symbol also goesthrough a Tomlinson-Harashima pre-coder (THP), which maps the PAM16 in-put in each dimension of the 4D symbolinto a quasi-continuous discrete-timevalue. Basically, it turns this signal intoa blob that is unintelligible to the hu-man eye, but the blob is imperviousto noise/interference because it prettymuch was turned into noise. The re-ceiver knows how to undo the THP, re-sulting in a clean bit stream. The benefitof this is it maximizes channel capacitywithout needing to increase the powerto get a good enough signal-to-noise ra-tio (SNR), or needing the receiver togain knowledge of the interference statethrough some sort of training.Conformance testingThe protocols and signaling that a de-vice needs to adhere to for the bestchance of interoperability are broad andencompass many layers of the Ethernetstack. However the 2.5/5GBase-T tech-nology is built on the robust foun-dation of prior Ethernet generationsand even at its infancy showed robustinteroperability.

First it leverages autonegotiation to
determine optimal link speed. In this
handshaking sequence devices must
send a series of link pulses to advertise their capabilities. Autonegotiation
is the Ethernet protocol that when correctly implemented, allows for a “plug
and play” scenario between any two
similarly capable devices. To ensure
higher-level protocols are correctly supported, a device also needs to adhere to
specific MAC and Flow Control protocol